Prior to the present invention, thermoplastic substrates, for example, polycarbonate articles, were treated with a photocurable acrylic coating composition, as shown in Moore et al., U.S. Pat. No. 4,198,465, assigned to the same assignee as the present invention. Although the use of cured polyacrylate coatings provided durable mar resistant and chemical resistant coatings, the ability of the cured polyacrylic coating often did not satisfy the abrasion resistant standards required in many applications.
As shown by Clark, U.S. Pat. No. 4,027,073, an acidic dispersion of colloidal silica in a hydroxylated silsesquioxane provides heat curable mixtures which result in valuable transparent abrasion resistant coatings on a variety of thermoplastic substrates. It was found, however, that the cure of the hydroxylated silsesquioxane binder resin often requires several hours, or even days to achieve acceptable mar resistant properties. Efforts to accelerate the cure speed of the aforementioned silsesquioxane colloidal silica coating compositions or impart improved abrasion resistance to the previously described polyacrylate coatings, have been unsuccessful.
The present invention is based on the discovery that a UV curable substantially solvent-free coating composition, based on the use of a UV photoinitiator, for example, .alpha.,.alpha.-diethoxyacetophenone, can be used in combination with a silyl acrylate of the formula, ##STR1## and aqueous colloidal silica, and optionally with acrylic monomer of the formula, ##STR2## to provide a UV curable organopolysiloxane hardcoat composition curable to an adherent abrasion resistant coating in several seconds or less, where R is a C.sub.(1-13) monovalent radical, R.sup.1 is a C.sub.(1-8) alkyl radical, R.sup.2 is selected from hydrogen and R radicals and mixtures thereof, R.sup.3 is a divalent C.sub.(1-8) alkylene radical and R.sup.4 is a polyvalent organic radical, a is a whole number equal to 0 to 2 inclusive, b is an integer equal to 1-3 inclusive, and the sum of a+b is equal 1 to 3 inclusive, n is an integer having a value of 2 to 4 inclusive.